More in detail, the present invention relates to a front or rear lamp for automobiles or motorcycles or similar motor vehicles, of the type comprising: a rear cup-shaped casing, being structured so as to be recessed inside of a compartment obtained in the vehicle body; a front lenticular body made from at least partially transparent or translucent materials and coupled to the rear casing in correspondence of the opening of the same so as to emerge from the vehicle body; and a LED lighting device that is designed to be housed inside the rear casing and comprises, in turn, one or more LED lighting circuits each provided with one or more LEDs arranged substantially facing the front lenticular body so as to perform an automotive lighting and/or signaling function, and an electronic control circuit having the purpose of controlling/adjusting the driving current which crosses the LEDs during operation thereof.
Some of the LED lighting devices described above are configured so as to supply a preset constant driving current to the LEDs. The driving current is determined during the lamp design step, and is dimensioned to allow, when in use, following the pre-heating, i.e. when the LEDs operate in the steady state condition, the light beam generated by the same to have a predetermined value. In particular, the driving current for supplying the LEDs is typically determined by a table of luminous/current flux contained in the LED datasheets provided by the LED manufacturers, on the basis of the luminous flux to be generated in a steady state condition.
It is known, however, that the luminous flux generated by a LED is not constant, but varies as a function of the junction temperature of the LED itself. In fact, the luminous flux generated by a LED is subject to a reduction/decay with increasing temperature of the LED itself. FIG. 1 illustrates an example of a graph showing the pattern of a LED luminous flux (shown in a normalized datasheet with respect to a temperature of 25° C.) supply by a constant rated current (400 mA) in response to temperature variation (in the temperature range comprised between −40° C. and 120° C.) in which the progressive linear reduction of the luminous flux with increasing temperature becomes evident. Some lighting devices have been implemented comprising electronic control circuits that provide the LEDs with a supply current determined on the basis of the required flux in the steady state condition associated with the achievement of a steady state condition associated with the attainment of a temperature typically within about 23° C. and 85° C. at the LED welding point or solder points.
However, said supply current can be over-dimensioned beyond the above mentioned temperature range. In fact, the electronic control circuits providing the LEDs with the supply current thus calculated have the technical problem of getting the LEDs to generate, in the preheating/initial ignition step, i.e. at temperatures relatively lower than the steady state temperature, a luminous flux that may be excessive. As shown in FIG. 1, at low temperature a LED supplied with the nominal driving current generates a high luminous flux as it is not yet considerably subjected to the luminous decay phenomenon. Obviously, supplying the LED with a driving current in the pre-heating step a resulting increase of the luminous flux is determined, the value of which, however, may be unacceptable in certain types of lamps, such as for example anti-fog lamps, as exceeding the maximum brightness thresholds set by the legal regulations governing the operation of said lamps. Obviously, this technical problem manifests itself in a particularly relevant way especially during the winter seasons and/or when the lamp operates in cold climates wherein the environment temperature can reach extremely low values, for example −40° C.
Therefore the need to obtain a lamp the flux of which is kept constant in response to change in the temperature has arisen. In particular, on the part of lamp manufacturers the need has arisen to provide a automotive lamp that is, on the one hand, suited to compensate for the luminous flux decay in a temperatures range comprised around a predetermined steady state temperature, higher than the LED preheating temperature and on the other hand, suited to attenuate in a controlled manner the luminous flux itself during the pre-heating step so as to keep it below tolerance thresholds according to the regulations, even when the outdoor environment operating temperature of the LED is relatively low.
EP 2 355 621 A2 describes a power supply circuit of a LED lamp structured to achieve energy savings during the LED pre-heating/lighting up step. The power supply circuit is essentially configured so as to control/adjust the current supplied to the LED according to a predetermined fixed linear function, which indicates the pattern of the current to be supplied to the LED as a function of the LED temperature. The linear function is represented graphically in EP 2 355 621 A2 by way of two rectilinear contiguous sections indicating the pattern of the current to be supplied to the LEDs in response to change in the temperature. When the LED temperature is lower than the steady state temperature, the power supply circuit increases in a linear way the current supplied to the LED on the basis of a temperature increase following the first rectilinear inclined section until reaching a predetermined current at the steady state temperature, while when the LED temperature exceeds the steady state temperature, the power supply circuit keeps the current constant as determined in the second horizontal section of the linear function.